Multi-piece solid golf ball

ABSTRACT

In a multi-piece solid golf ball having a core, an envelope layer, an intermediate layer and a cover, the hardness profile at the core interior and the hardness relationships among the various layers are optimized. The ball is endowed with an excellent flight performance on shots with a driver and an excellent controllability in the short game that are acceptable to professionals and other skilled golfers. The ball also has a good feel at impact and an excellent scuff resistance.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of copending application Ser.No. 12/912,386 filed on Oct. 26, 2010, the entire contents of which arehereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a multi-piece solid golf ball composedof a core, an envelope layer, an intermediate layer and a cover thathave been formed as successive layers. More specifically, the inventionrelates to a multi-piece solid golf ball which, when used by thosegolfers among professionals and skilled amateurs who, on striking a ballwith a driver, tend to generate shots having a rather low spin rate anda low launch angle, has an excellent flight performance and an excellentcontrollability in the short game that are acceptable to such users, andwhich moreover has both a good feel at impact and an excellent scuffresistance.

Key performance features required in a golf ball include distance,controllability, durability and feel. Balls having these qualities inthe highest degree are constantly being sought. Among recent golf balls,a succession of balls having multi-piece structures which are typicallycomposed of three pieces have emerged. By having the structure of a golfball be multilayered, it is possible to combine many materials ofdifferent properties, thus enabling a wide variety of ball designs inwhich each layer has a particular function.

In particular, multi-piece solid golf balls having an optimized hardnessrelationship among the respective layers encasing the core, such as theintermediate layer and cover, are widely used. Recently, in addition tothe flight performance of a ball, the durability of the ball to repeatedimpact (which inhibits crack formation) and the scuff resistance (whichinhibits burr formation on the ball surface) have also become importantfactors in evaluating ball performance. Therefore, a major challenge isto design the thickness and hardness of the respective ball layers insuch a way as to maximize these effects.

With regard to golf balls for professionals and other skilled golfers inparticular, there exists a desire for the development of balls in whichthe thickness and hardness of each layer encasing the core, such as theintermediate layer and the cover layer, have been highly optimized inorder not only to provide the ball with a good feel and excellentdurability but also to achieve both a superior distance performance inthe high head speed range and precise controllability on shots with aniron and on approach shots.

Golf balls with such a multilayer structure have been disclosed in, forexample, JP-A 2009-160407, U.S. Pat. No. 6,302,808, JP-A 2001-017569,JP-A 2001-017570, JP-A 2001-037914, JP-A 2008-149131, JP-A 2009-095365and JP-A 2009-095369. However, further improvements in spin rate, feelat impact, and control of the initial velocity are desired.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide amulti-piece solid golf ball which has a flight performance andcontrollability that are acceptable to professionals and other skilledgolfers, is capable in particular of exhibiting an excellentcontrollability even in the short game, has a good feel at impact, andalso has an excellent scuff resistance.

The inventors have conducted extensive investigations in order to attainthe above object. As a result, they have discovered that, by optimizingthe hardness profile at the core interior and adopting a basic ballconstruction wherein the layers encasing the core have a multilayerstructure of five or more layers which includes, in order from the innerside: an inner envelope layer, an intermediate envelope layer, an outerenvelope layer, an intermediate layer and a cover, and by optimizing thehardness relationship among these various layers in such a way that thematerial hardness of the cover is the same as or lower than the averagehardness of the core, and the material hardness of one layer from amongthe envelope layers and the intermediate layer (which layers aresometimes collectively referred to below as the “inner layers”) ishigher than the material hardness of the cover and/or the average corehardness (defined as the arithmetic mean of the core center hardness andthe core surface hardness), there can be obtained a golf ball which hasa flight performance and controllability that are fully satisfactory onshots with a driver, which has an excellent flight performance andcontrollability even in the short game, and which moreover has anexcellent scuff resistance and a good feel at impact.

Accordingly, the invention provides the following multi-piece solid golfball.

[1] A multi-piece solid golf ball comprising a core, an envelope layerencasing the core, an intermediate layer encasing the envelope layer,and a cover which encases the intermediate layer and has formed on asurface thereof a plurality of dimples, wherein the envelope layer iscomprised of an inner envelope layer, an intermediate envelope layer andan outer envelope layer; the inner, intermediate and outer envelopelayers, the intermediate layer and the cover are each formed primarilyof a resin material which may be of the same or different types; thecore is formed primarily of a rubber material; the cover has a materialhardness (Shore D) and the core has a center hardness (Shore D) and asurface hardness (Shore D) with an arithmetic average thereof (averagecore hardness), which cover material hardness and average core hardnesssatisfy the following condition:

cover material hardness average core hardness; one layer from among theenvelope layers and the intermediate layer has a material hardness(Shore D) which is higher than either or both of the cover materialhardness (Shore D) and the average core hardness; and, letting C10represent a JIS-C cross-sectional hardness at a position 10 mm from acenter of the core on a cross-section obtained by cutting the core inhalf and H represent a JIS-C surface hardness of the core, thehardnesses C10 and H satisfy the following condition:

10≦(H−C10)≦30.

[2] The multi-piece solid golf ball of [1], wherein the intermediateenvelope layer is formed so as to be harder than the inner envelopelayer and to have a material hardness difference (Shore D) with theinner envelope layer of from 1 to 10, and so as to be softer than theouter envelope layer and to have a material hardness difference (ShoreD) with the outer envelope layer of from 1 to 10.[3] The multi-piece solid golf ball of [1], wherein the intermediatelayer and the cover have thicknesses which satisfy the followingcondition:

1.3≦intermediate layer thickness/cover thickness≦4.0.

[4] The multi-piece solid golf ball of [1], wherein the inner envelopelayer, intermediate envelope layer and outer envelope layer havethicknesses which satisfy the following condition:

inner envelope layer thickness≦intermediate envelope

layer thickness≦outer envelope layer thickness.

[5] The multi-piece solid golf ball of [1], wherein the intermediatelayer is formed of a material which includes an ionomer resin having anacid content of at least 16 wt %.[6] The multi-piece solid golf ball of [1], wherein the core center,outer envelope layer, intermediate layer and cover have hardnesses(Shore D) which satisfy the following condition:

cover material hardness<intermediate layer material hardness>outerenvelope layer material hardness>core center hardness.

[7] The multi-piece solid golf ball of [1], wherein the core center,inner envelope layer, intermediate envelope layer, outer envelope layer,intermediate layer and cover have hardnesses (Shore D) which satisfy thefollowing condition:

cover material hardness<intermediate layer material hardness>outerenvelope layer material hardness>intermediate envelope layer materialhardness>inner envelope layer material hardness>core center hardness.

[8] The multi-piece solid golf ball of [1], wherein the core, innerenvelope layer, intermediate envelope layer, outer envelope layer,intermediate layer and cover have thicknesses which satisfy thefollowing condition:

cover thickness<intermediate layer thickness<(outer envelope layerthickness+intermediate envelope layer thickness+inner envelope layerthickness)<core diameter.

[9] The multi-piece solid golf ball of [1], wherein the inner envelopelayer, intermediate envelope layer, outer envelope layer, intermediatelayer and cover have thicknesses which satisfy the following condition:

(cover thickness+intermediate layer thickness)<(outer envelope layerthickness+intermediate envelope layer thickness+inner envelope layerthickness).

[10] The multi-piece solid golf ball of [1], wherein at least one layerfrom among the inner envelope layer, intermediate envelope layer andouter envelope layer is formed of a material obtained by blending:

an ionomer resin component of (a) an olefin-unsaturated carboxylic acidrandom copolymer and/or a metal ion neutralization product of anolefin-unsaturated carboxylic acid random copolymer mixed with (b) anolefin-unsaturated carboxylic acid-unsaturated carboxylic acid esterrandom terpolymer and/or a metal ion neutralization product of anolefin-unsaturated carboxylic acid-unsaturated carboxylic acid esterrandom terpolymer in a weight ratio between 100:0 and 0:100, and

(e) a non-ionomeric thermoplastic elastomer in a weight ratio between100:0 and 50:50.

[11] The multi-piece solid golf ball of [1], wherein at least one layerfrom among the inner envelope layer, intermediate envelope layer andouter envelope layer is formed of a material obtained by blending asessential components:

100 parts by weight of a resin component composed of, in admixture,

-   -   a base resin of (a) an olefin-unsaturated carboxylic acid random        copolymer and/or a metal ion neutralization product of an        olefin-unsaturated carboxylic acid random copolymer mixed        with (b) an olefin-unsaturated carboxylic acid-unsaturated        carboxylic acid ester random terpolymer and/or a metal ion        neutralization product of an olefin-unsaturated carboxylic        acid-unsaturated carboxylic acid ester random terpolymer in a        weight ratio between 100:0 and 0:100, and    -   (e) a non-ionomeric thermoplastic elastomer in a weight ratio        between 100:0 and 50:50;

(c) from 5 to 120 parts by weight of a fatty acid and/or fatty acidderivative having a molecular weight of from 228 to 1500; and

(d) from 0.1 to 17 parts by weight of a basic inorganic metal compoundcapable of neutralizing un-neutralized acid groups in the base resin andcomponent (c).

[12] The multi-piece solid golf ball of [11] wherein at least two layersfrom among the inner envelope layer, intermediate envelope layer andouter envelope layer are formed of the material of [11].[13] The multi-piece solid golf ball of [11] wherein the inner envelopelayer, intermediate envelope layer and outer envelope layer are allformed of the material of [11].[14] The multi-piece solid golf ball of [1], wherein the core has adeflection when compressed under a final load of 1,275 N (130 kgf) froman initial load state of 98 N (10 kgf) of at least 1.8 mm but not morethan 6.0 mm.[15] The multi-piece solid golf ball of [1], wherein the cover is formedby injection molding a resin blend composed primarily of (A) athermoplastic polyurethane and (B) a polyisocyanate compound, whichresin blend contains a polyisocyanate compound in at least some portionof which all the isocyanate groups remain in an unreacted state.[16] The multi-piece solid golf ball of [1], wherein, letting C0represent a JIS-C cross-sectional hardness at the center of the core ona cross-section obtained by cutting the core in half, C4 represent aJIS-C cross-sectional hardness at a position 4 mm from the center of thecore, C10 represent a JIS-C cross-sectional hardness at a position 10 mmfrom the center of the core, and H represent a JIS-C surface hardness ofthe core, the hardnesses C0, C4, C10 and H satisfy the conditions:

C4−C0≦5,

3≦C10−C4≦12, and

C4−C0<C10−C4<H−C10.

[17] The multi-piece solid golf ball of [1], wherein the core has asingle-layer structure.

BRIEF DESCRIPTION OF THE DIAGRAMS

FIG. 1 is a schematic sectional view showing a multi-piece solid golfball (six-layer construction) according to the invention.

FIG. 2 is a top view showing the dimple pattern used on the balls in theexamples.

DETAILED DESCRIPTION OF THE INVENTION

The invention is described more fully below. The multi-piece solid golfball of the present invention is, as shown in FIG. 1, a golf ball G witha six-layer construction that includes a core 1, an inner envelope layer2 a, intermediate envelope layer 2 b and outer envelope layer 2 c whichencase the core 1, an intermediate layer 3 which encases the envelopelayers, and a cover 4 which encases the intermediate layer 3. The cover4 typically has a large number of dimples D formed on the surfacethereof. The core 1, the intermediate layer 3 and the cover 4 are notlimited to single layers, and may each be formed of a plurality of twomore layers.

In the invention, the core has a diameter which, although not subject toany particular limitation, is preferably at least 20 mm, more preferablyat least 22 mm, and even more preferably at least 24 mm. There is noparticular upper limit in the diameter, although the diameter ispreferably not more than 35 mm, more preferably not more than 30 mm, andeven more preferably not more than 28 mm. At a core diameter outsidethis range, the ball may have a lower initial velocity and it may not bepossible to obtain a suitable spin rate following impact, as a result ofwhich an increased distance may not be achieved.

In the practice of the invention, by optimizing the hardness profile atthe interior of the core, an even lower spin rate can be achieved onshots with a driver, enabling the ball to travel an increased distance.A detailed description of the hardness profile at the interior of thecore follows.

First, in the invention, to set the hardnesses at various places on theinterior of the core, let C0 represent a JIS-C cross-sectional hardnessat a center of the core on a cross-section obtained by cutting the corein half, C2 represent a JIS-C cross-sectional hardness at a position 2mm from the core center, C4 represent a JIS-C cross-sectional hardnessat a position 4 mm from the core center, C6 represent a JIS-Ccross-sectional hardness at a position 6 mm from the core center, C8represent a JIS-C cross-sectional hardness at a position 8 mm from thecore center, C10 represent a JIS-C cross-sectional hardness at aposition 10 mm from the core center, C12 represent a JIS-Ccross-sectional hardness at a position 12 mm from the core center, and Hrepresent a JIS-C surface hardness of the core. The hardnesses at therespective places on the core cross-section are described in detailbelow.

The core surface hardness H, although not subject to any particularlimitation, has a JIS-C hardness value of preferably at least 75, morepreferably at least 80, and even more preferably at least 84. The upperlimit in the JIS-C value is preferably not more than 100, morepreferably not more than 95, and even more preferably not more than 92.The above hardness range, when expressed as the Shore D hardness, ispreferably at least 49, more preferably at least 53, and even morepreferably at least 56. The upper limit in the Shore D hardness ispreferably not more than 68, more preferably not more than 64, and evenmore preferably not more than 62.

The core center hardness C0, although not subject to any particularlimitation, has a JIS-C hardness value of preferably at least 55, morepreferably at least 57, and even more preferably at least 60. The upperlimit in the JIS-C value is preferably not more than 80, more preferablynot more than 70, and even more preferably not more than 65. The abovehardness range, when expressed as the Shore D hardness, is preferably atleast 34, more preferably at least 35, and even more preferably at least38. The upper limit in the Shore D hardness is preferably not more than53, more preferably not more than 45, and even more preferably not morethan 41.

The core cross-sectional hardness C2, although not subject to anyparticular limitation, has a JIS-C hardness value of preferably at least55, more preferably at least 57, and even more preferably at least 60.There is no particular upper limit, although the JIS-C hardness value ispreferably not more than 80, more preferably not more than 70, and evenmore preferably not more than 65.

The core cross-sectional hardness C4, although not subject to anyparticular limitation, has a JIS-C hardness value of preferably at least55, more preferably at least 57, and even more preferably at least 60.There is no particular upper limit, although the JIS-C hardness value ispreferably not more than 80, more preferably not more than 70, and evenmore preferably not more than 65.

The core cross-sectional hardness C6, although not subject to anyparticular limitation, has a JIS-C hardness value of preferably at least56, more preferably at least 58, and even more preferably at least 61.There is no particular upper limit, although the JIS-C hardness value ispreferably not more than 81, more preferably not more than 71, and evenmore preferably not more than 66.

The core cross-sectional hardness C8, although not subject to anyparticular limitation, has a JIS-C hardness value of preferably at least58, more preferably at least 60, and even more preferably at least 63.There is no particular upper limit, although the JIS-C hardness value ispreferably not more than 83, more preferably not more than 73, and evenmore preferably not more than 68.

The core cross-sectional hardness C10, although not subject to anyparticular limitation, has a JIS-C hardness value of preferably at least60, more preferably at least 62, and even more preferably at least 65.There is no particular upper limit, although the JIS-C hardness value ispreferably not more than 85, more preferably not more than 75, and evenmore preferably not more than 70.

The core cross-sectional hardness C12, although not subject to anyparticular limitation, has a JIS-C hardness value of preferably at least63, more preferably at least 65, and even more preferably at least 68.There is no particular upper limit, although the JIS-C hardness value ispreferably not more than 88, more preferably not more than 78, and evenmore preferably not more than 73.

In the invention, it is critical for the hardness difference H−C10between the surface hardness H and the cross-sectional hardness C10 tohave a JIS-C hardness value of from 10 to 30. This hardness differencehas a lower limit of preferably at least 13, and more preferably atleast 16, and has an upper limit of preferably not more than 27, andmore preferably not more than 24. If the hardness difference is toosmall, the spin rate on shots with a driver (W#1) may rise, as a resultof which a good distance may not be achieved. On the other hand, if thehardness difference is too large, the durability of the ball to crackingon repeated impact may worsen.

The hardness difference C4−C0 between the cross-sectional hardness C4and the center hardness C0, although not subject to any particularlimitation, may be set to a JIS-C hardness value of preferably not morethan 5, more preferably not more than 4, and even more preferably notmore than 3. There is no particular lower limit, although the JIS-Chardness value may be set to preferably at least 0. If the hardnessdifference is too large or too small, the spin rate on shots with adriver (W#1) may rise, as a result of which a good distance may not beachieved.

The hardness difference C10−C4 between the cross-sectional hardness C10and the cross-sectional hardness C4, although not subject to anyparticular limitation, may be set to a JIS-C hardness value ofpreferably at least 3, more preferably at least 4, and even morepreferably at least 5. There is no particular upper limit, although theJIS-C hardness value may be set to preferably not more than 12, morepreferably not more than 10, and even more preferably not more than 8.If the hardness difference is too small, the spin rate on shots with adriver (W#1) may rise, as a result of which a good distance may not beachieved. On the other hand, if the hardness difference is too large,the durability of the ball to cracking on repeated impact may worsen.

The hardness difference H−C0 between the core center hardness C0 and thecore surface hardness H, although not subject to any particularlimitation, may be set to a JIS-C hardness value of preferably at least13, more preferably at least 18, and even more preferably at least 22.There is no particular upper limit, although the JIS-C hardness valuemay be set to preferably not more than 35, more preferably not more than33, and even more preferably not more than 31. If the hardnessdifference is too small, the spin rate on shots with a driver (W#1) maybecome too high, as a result of which an increased distance may not beachieved. On the other hand, if the hardness difference is too large,the durability of the ball to cracking on repeated impact may worsen orthe rebound may decrease, as a result of which an increased distance maynot be achieved.

Moreover, it is preferable, although not critical, for the various abovehardness differences to satisfy the following condition:

C4−C0<C10−C4<H−C10.

If the above condition is not satisfied, the spin rate of the ball onshots with a driver (W#1) may rise, as a result of which a good distancemay not be achieved.

It is essential for the arithmetic mean of the core surface hardness Hand the core center hardness C0 (which arithmetic mean is referred tobelow as the “average core hardness”) to be equal to or higher than thesubsequently described cover material hardness. Here, the range in theaverage core hardness, expressed as the JIS-C hardness, may be set topreferably at least 65, more preferably at least 68, and even morepreferably at least 72. The upper limit, expressed as the JIS-Chardness, is preferably not more than 90, more preferably not more than88, and even more preferably not more than 78. The above hardness range,expressed as the Shore D hardness, is preferably at least 41, morepreferably at least 44, and even more preferably at least 47. The upperlimit is preferably not more than 60, more preferably not more than 59,and even more preferably not more than 51.

At an average core hardness below the above ranges, the core may have aninadequate resilience, as a result of which an increased distance maynot be achieved; also, the feel of the ball at impact may be too soft,and the durability of the ball to cracking on repeated impact mayworsen. Conversely, at an average core hardness higher than the aboverange, the ball may have an excessively hard feel on full shots and thespin rate may be too high, as a result of which an increased distancemay not be achieved.

The core has a deflection when subjected to compressive loading, i.e.,when compressed under a final load of 1,275 N (130 kgf) from an initialload state of 98 N (10 kgf), which, while not subject to any particularlimitation, is preferably at least 1.8 mm, more preferably at least 2.0mm, and even more preferably at least 2.4 mm. The upper limit, althoughnot subject to any particular limitation, is preferably not more than6.0 mm, more preferably not more than 5.0 mm, and even more preferablynot more than 3.5 mm. If this value is too high, the resilience of thecore may become too low, resulting in an insufficient distance, the feelmay become too soft, or the durability of the ball to cracking onrepeated impact may worsen. On the other hand, if this value is too low,the ball may have an excessively hard feel on full shots, or the spinrate may be too high, as a result of which an increased distance may notbe achieved.

A material composed primarily of rubber may be used to form the corehaving the above-described surface hardness and deflection. For example,the core may be formed of a rubber composition containing, in additionto the rubber component, a co-crosslinking agent, sulfur, an organicperoxide, an inert filler and an organosulfur compound. Polybutadiene ispreferably used as the base rubber of this rubber composition.

It is desirable for the polybutadiene to have a cis-1,4 bond content onthe polymer chain of at least 60 wt %, preferably at least 80 wt %, morepreferably at least 90 wt %, and most preferably at least 95 wt %. Toolow a cis-1,4 bond content among the bonds on the molecule may result ina lower resilience.

Also, the polybutadiene has a 1,2-vinyl bond content on the polymerchain of typically not more than 2%, preferably not more than 1.7%, andmore preferably not more than 1.5%. Too high a 1,2-vinyl bond contentmay result in a lower resilience.

To obtain a molded and vulcanized rubber composition of good resilience,the polybutadiene is preferably one synthesized with a rare-earthcatalyst or a Group VIII metal compound catalyst. Polybutadienesynthesized with a rare-earth catalyst is especially preferred.

Such rare-earth catalysts are not subject to any particular limitation.Exemplary rare-earth catalysts include those made up of a combination ofa lanthanide series rare-earth compound with an organoaluminum compound,an alumoxane, a halogen-bearing compound and an optional Lewis base.

Examples of suitable lanthanide series rare-earth compounds includehalides, carboxylates, alcoholates, thioalcoholates and amides of atomicnumber 57 to 71 metals.

In the practice of the invention, the use of a neodymium catalyst inwhich a neodymium compound serves as the lanthanide series rare-earthcompound is particularly advantageous because it enables a polybutadienerubber having a high cis-1,4 bond content and a low 1,2-vinyl bondcontent to be obtained at an excellent polymerization activity. Suitableexamples of such rare-earth catalysts include those mentioned in JP-A11-35633, JP-A 11-164912 and JP-A 2002-293996.

To enhance the resilience, it is preferable for the polybutadienesynthesized using the lanthanide series rare-earth compound catalyst toaccount for at least 10 wt %, preferably at least 20 wt %, and morepreferably at least 40 wt %, of the rubber components.

Rubber components other than the above-described polybutadiene may beincluded in the base rubber insofar as the objects of the invention areattainable. Illustrative examples of rubber components other than theabove-described polybutadiene include other polybutadienes, and otherdiene rubbers, such as styrene-butadiene rubber, natural rubber,isoprene rubber and ethylene-propylene-diene rubber.

Examples of co-crosslinking agents include unsaturated carboxylic acidsand the metal salts of unsaturated carboxylic acids.

Specific examples of unsaturated carboxylic acids include acrylic acid,methacrylic acid, maleic acid and fumaric acid. Acrylic acid andmethacrylic acid are especially preferred.

The metal salts of unsaturated carboxylic acids, while not subject toany particular limitation, are exemplified by the above-mentionedunsaturated carboxylic acids neutralized with a desired metal ion.Specific examples include the zinc and magnesium salts of methacrylicacid and acrylic acid. The use of zinc acrylate is especially preferred.

The amount of unsaturated carboxylic acid and/or metal salt thereofincluded per 100 parts by weight of the base rubber may be set topreferably at least 10 parts by weight, more preferably at least 15parts by weight, and even more preferably at least 20 parts by weight.The upper limit may be set to preferably not more than 60 parts byweight, more preferably not more than 50 parts by weight, even morepreferably not more than 45 parts by weight, and most preferably notmore than 40 parts by weight. Too much may make the core too hard,giving the ball an unpleasant feel at impact, whereas too little maylower the rebound.

The sulfur is not subject to any particular limitation, althoughpreferred use may be made of a known powdered sulfur or the like. Theamount of the sulfur included per 100 parts by weight of the base rubbermay be set to preferably at least 0.01 part by weight, more preferablyat least 0.03 part by weight, and even more preferably at least 0.05part by weight. The upper limit may be set to preferably not more than0.3 part by weight, more preferably not more than 0.2 part by weight,and even more preferably not more than 0.1 part by weight. At an amountoutside of the above range, a suitable hardness profile at the coreinterior may not be obtained, as a result of which the spin rate mayrise, as a result of which a good distance may not be achieved, and thedurability to cracking on repeated impact may worsen.

A known organic peroxide may be used as the organic peroxide.Illustrative examples include dicumyl peroxide,1,1-di(t-butylperoxy)cyclohexane, t-butylperoxy laurate, dibenzoylperoxide, dilauroyl peroxide and1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane. These may be usedsingly or two or more may be used in combination. Commercial productsmay be used as these organic peroxides. Illustrative examples of suchcommercial products include those available under the trade names“Percumyl D,” “Perhexa C-40,” “Perbutyl L” (all from NOF Corporation),the trade names “Niper BW” and “Peroyl L” (both from NOF Corporation),and the trade name “Trigonox 29” (from Kayaku Akzo Corporation).

The organic peroxide is included in an amount, per 100 parts by weightof the base rubber, of preferably at least 0.2 part by weight, morepreferably at least 0.4 part by weight, and even more preferably atleast 0.6 part by weight. The upper limit is preferably not more than 5parts by weight, more preferably not more than 3 parts by weight, evenmore preferably not more than 2 parts by weight. At an amount outside ofthe above range, a suitable hardness profile at the core interior maynot be obtained, as a result of which the spin rate on shots with adriver (W#1) may rise, as a result of which a good distance may not beachieved, and the durability to cracking on repeated impact may worsen.

The compounding ratio based on weight between the above sulfur and theabove organic peroxide (sulfur/organic peroxide), although notparticularly limited, may be set to preferably at least 0.02, morepreferably at least 0.04, and even more preferably at least 0.06. Theupper limit may be set to preferably not more than 0.5, more preferablynot more than 0.3, and even more preferably not more than 0.2. At acompounding ratio outside of the above range, a suitable hardnessprofile may not be obtained, as a result of which the spin rate on shotswith a driver (W#1) may rise, as a result of which a good distance maynot be achieved, and the durability to cracking on repeated impact mayworsen.

Examples of suitable inert fillers include zinc oxide, barium sulfateand calcium carbonate. These may be used singly or as a combination oftwo or more thereof.

The amount of inert filler included per 100 parts by weight of the baserubber may be set to preferably at least 1 part by weight, and morepreferably at least 5 parts by weight. The upper limit may be set topreferably not more than 200 parts by weight, more preferably not morethan 150 parts by weight, and even more preferably not more than 100parts by weight. Too much or too little inert filler may make itimpossible to achieve a proper weight and a good rebound.

In addition, an antioxidant may be included if necessary. Illustrativeexamples of suitable commercial antioxidants include Nocrac NS-6, NocracNS-30 (both available from Ouchi Shinko Chemical Industry Co., Ltd.),and Yoshinox 425 (Yoshitomi Pharmaceutical Industries, Ltd.). These maybe used singly or as a combination of two or more thereof.

The amount of antioxidant included may be more than 0, and is set topreferably at least 0.05 part by weight, and especially at least 0.1part by weight, per 100 parts by weight of the base rubber. The upperlimit, although not subject to any particular limitation, may be set topreferably not more than 3 parts by weight, more preferably not morethan 2 parts by weight, even more preferably not more than 1 part byweight, and most preferably not more than 0.5 part by weight, per 100parts by weight of the base rubber. Too much or too little antioxidantmay make it impossible to achieve a good rebound and durability.

To enhance the rebound of the golf ball and increase its initialvelocity, it is preferable to include an organosulfur compound in theabove base rubber. No particular limitation is imposed on theorganosulfur compound, provided it improves the rebound of the golfball. Exemplary organosulfur compounds include thiophenols,thionaphthols, halogenated thiophenols, and metal salts thereof.Specific examples include pentachlorothiophenol, pentafluorothiophenol,pentabromothiophenol, p-chlorothiophenol, the zinc salt ofpentachlorothiophenol, the zinc salt of pentafluorothiophenol, the zincsalt of pentabromothiophenol, the zinc salt of p-chlorothiophenol; anddiphenylpolysulfides, dibenzylpolysulfides, dibenzoylpolysulfides,dibenzothiazoylpolysulfides and dithiobenzoylpolysulfides having 2 to 4sulfurs. The zinc salt of pentachlorothiophenol is especially preferred.

The amount of such an organosulfur compound included per 100 parts byweight of the base rubber may be set to preferably at least 0.05 part byweight, more preferably at least 0.1 part by weight, and even morepreferably at least 0.2 part by weight. It is recommended that the upperlimit in the amount of the organosulfur compound included per 100 partsby weight of the base rubber be preferably not more than 5 parts byweight, more preferably not more than 3 parts by weight, and even morepreferably not more than 2.5 parts by weight. If too much organosulfurcompound is included, further improvement in the rebound (especiallywhen struck with a W#1) is unlikely to be achieved and the core maybecome too soft, possibly resulting in a poor feel.

Next, the envelope layer is described.

In the present invention, as noted above, the envelope layer encasingthe core is formed of three layers: an inner envelope layer, anintermediate envelope layer, and an outer envelope layer.

The inner envelope layer has a material hardness, expressed as the ShoreD hardness (measured with a type D durometer in general accordance withASTM D 2240), which, while not subject to any particular limitation, ispreferably at least 38, more preferably at least 40, and even morepreferably at least 43. The upper limit, although not subject to anyparticular limitation, is preferably not more than 60, more preferablynot more than 55, and even more preferably not more than 50. It ispreferable for the inner envelope layer to be formed so as to be softerthan the intermediate envelope layer. If the inner envelope layer is toosoft, a loss of energy may arise on full shots, lowering the initialvelocity, as a result of which an increased distance may not beachieved. On the other hand, if the inner envelope layer is too hard,the durability of the ball to cracking under repeated impact may worsenor the ball may have too hard a feel when played. As used herein,“material hardness” refers to, in cases where the material is a resin,the measured hardness of a 2 mm thick sheet produced by molding theresin composition under applied pressure. In cases where the material isa rubber, the “material hardness” refers to the measured hardness of apressed sheet having a thickness of about 2 mm produced by loading therubber composition into a sheet-forming mold and hot molding at 170° C.for 15 minutes (the same applies below).

The inner envelope layer has a thickness which, although not subject toany particular limitation, is preferably at least 0.5 mm, morepreferably at least 0.7 mm, and even more preferably at least 0.9 mm.The upper limit, although not subject to any particular limitation, ispreferably not more than 3.5 mm, more preferably not more than 2.5 mm,and even more preferably not more than 2.0 mm. At an inner envelopelayer thickness outside this range, the spin rate-lowering effect onshots with a driver (W#1) may be inadequate, as a result of which anincreased distance may not be achieved.

The intermediate envelope layer encasing the inner envelope layer has amaterial hardness, expressed as the Shore D hardness, which, althoughnot subject to any particular limitation, is preferably at least 40,more preferably at least 45, and even more preferably at least 47. Theupper limit, although not subject to any particular limitation, ispreferably not more than 62, more preferably not more than 58, and evenmore preferably not more than 55. If the intermediate envelope layer istoo soft, the ball may have a low initial velocity on full shots, as aresult of which an increased distance may not be achieved. On the otherhand, if the intermediate envelope layer is too hard, the durability ofthe ball to cracking on repeated impact may worsen or the ball may havetoo hard a feel when played.

In the present invention, it is preferable for the intermediate envelopelayer to be formed so as to be harder than the inner envelope layer andsofter than the outer envelope layer. In this case, although not subjectto any particular limitation, the hardness difference between theintermediate envelope layer and the inner envelope layer, expressed interms of the Shore D hardness, is set to a value of preferably at least1, more preferably at least 2, and even more preferably at least 3. Theupper limit, although not subject to any particular limitation, is setto preferably not more than 10, more preferably not more than 5, andeven more preferably not more than 4. Likewise, the hardness differencebetween the intermediate envelope layer and the outer envelope layer,expressed in terms of the Shore D hardness, is set to a value ofpreferably at least 1, more preferably at least 2, and even morepreferably at least 3. The upper limit, although not subject to anyparticular limitation, is set to preferably not more than 10, morepreferably not more than 5, and even more preferably not more than 4. Ifthe inner and outer envelope layers adjoining the intermediate envelopelayer do not satisfy the above hardness relationships or the hardnessdifferences do not fall within the above ranges, a loss of energy mayarise on full shots, lowering the initial velocity, as a result of whichan increased distance may not be achieved.

The intermediate envelope layer has a thickness which, although notsubject to any particular limitation, is preferably at least 0.8 mm,more preferably at least 1.2 mm, and even more preferably at least 1.7mm. The upper limit, although not subject to any particular limitation,is preferably not more than 3.8 mm, more preferably not more than 3.2mm, and even more preferably not more than 2.7 mm. At an intermediateenvelope layer thickness outside this range, the spin rate-loweringeffect on shots with a driver (W#1) may be inadequate, as a result ofwhich an increased distance may not be achieved, or the feel at impactmay worsen.

The outer envelope layer encasing the intermediate envelope layer has amaterial hardness, expressed as the Shore D hardness, which, althoughnot subject to any particular limitation, is preferably at least 42,more preferably at least 49 and even more preferably at least 51. Theupper limit, although not subject to any particular limitation, ispreferably not more than 65, more preferably not more than 62, and evenmore preferably not more than 60. Also, the outer envelope layer ispreferably formed so as to be softer than the subsequently describedintermediate layer. If the outer envelope layer is too soft, thehardness difference with the intermediate layer may be too large, givingrise to a loss of energy on full shots, as a result of which anincreased distance may not be achieved. On the other hand, if the outerenvelope layer is too hard, the durability of the ball to cracking onrepeated impact may worsen or the ball may have too hard a feel whenplayed.

The outer envelope layer has a thickness which, although not subject toany particular limitation, is preferably at least 1.0 mm, morepreferably at least 1.5 mm, and even more preferably at least 2.0 mm.The upper limit, although not subject to any particular limitation, ispreferably not more than 4.0 mm, more preferably not more than 3.5 mm,and even more preferably not more than 3.0 mm. At an outer envelopelayer thickness outside this range, the initial velocity on shots with adriver (W#1) may be inadequate, as a result of which an increaseddistance may not be achieved, or the feel at impact may worsen.

The combined thickness of the inner envelope layer, intermediateenvelope layer and outer envelope layer, i.e., the total thickness ofthe envelope layers, although not subject to any particular limitation,is preferably at least 2.3 mm, more preferably at least 3.4 mm, and evenmore preferably at least 4.6 mm. The upper limit, although not subjectto any particular limitation, is preferably not more than 11.3 mm, morepreferably not more than 9.2 mm, and even more preferably not more than7.7 mm. At a total thickness for the envelope layers outside the aboverange, the initial velocity on shots with a driver (W#1) may beinadequate, as a result of which a sufficient distance may not beachieved, or the solid feel at impact that tells the high head-speedgolfer that the ball is taking off may not be obtained.

In the present invention, the envelope layer is composed of threelayers—an inner envelope layer, an intermediate envelope layer and anouter envelope layer, which respective layers may be made of the same ormutually differing resin materials. The materials which form theseenvelope layers may be, for example, rubber materials or resinmaterials, and are not subject to any particular limitation. However, inthis invention, preferred use may be made of a material which includesas an essential component a base resin composed of, in admixture,specific amounts of (a) an olefin-unsaturated carboxylic acid randomcopolymer and/or a metal ion neutralization product of anolefin-unsaturated carboxylic acid random copolymer, and (b) anolefin-unsaturated carboxylic acid-unsaturated carboxylic acid esterrandom terpolymer and/or a metal ion neutralization product of anolefin-unsaturated carboxylic acid-unsaturated carboxylic acid esterrandom terpolymer. In the invention, by using this material to form atleast one of the envelope layers, a high rebound on shots with a driver(W#1) can be obtained, enabling a longer distance to be achieved. Thismaterial is described in detail below.

The olefin in the above base resin, whether in component (a) orcomponent (b), has a number of carbons which is generally at least 2 butnot more than 8, and preferably not more than 6. Specific examplesinclude ethylene, propylene, butene, pentene, hexene, heptene andoctene. Ethylene is especially preferred.

Examples of unsaturated carboxylic acids include acrylic acid,methacrylic acid, maleic acid and fumaric acid. Acrylic acid andmethacrylic acid are especially preferred.

Moreover, the unsaturated carboxylic acid ester is preferably a loweralkyl ester of the above unsaturated carboxylic acid. Specific examplesinclude methyl methacrylate, ethyl methacrylate, propyl methacrylate,butyl methacrylate, methyl acrylate, ethyl acrylate, propyl acrylate andbutyl acrylate. Butyl acrylate (n-butyl acrylate, i-butyl acrylate) isespecially preferred.

The olefin-unsaturated carboxylic acid random copolymer of component (a)and the olefin-unsaturated carboxylic acid-unsaturated carboxylic acidester random terpolymer of component (b) (the copolymers in components(a) and (b) are referred to collectively below as “random copolymers”)can each be obtained by random copolymerization of the above componentsusing a known method.

It is recommended that the above random copolymers have unsaturatedcarboxylic acid contents (acid contents) which are regulated. Here, itis recommended that the content of unsaturated carboxylic acid presentin the random copolymer serving as component (a), although not subjectto any particular limitation, be set to preferably at least 4 wt %, morepreferably at least 6 wt %, even more preferably at least 8 wt %, andmost preferably at least 10 wt %. Also, it is recommended that the upperlimit, although not subject to any particular limitation, be preferablynot more than 30 wt %, more preferably not more than 20 wt %, even morepreferably not more than 18 wt %, and most preferably not more than 15wt %.

Similarly, the content of unsaturated carboxylic acid present in therandom copolymer serving as component (b), although not subject to anyparticular limitation, may be set to preferably at least 4 wt %, morepreferably at least 6 wt %, and even more preferably at least 8 wt %.Also, it is recommended that the upper limit, although not subject toany particular limitation, be preferably not more than 15 wt %, morepreferably not more than 12 wt %, and even more preferably not more than10 wt %. If the acid content of the random copolymer is too low, theresilience may decrease, whereas if it is too high, the processabilitymay decrease.

The metal ion neutralization products of the random copolymers ofcomponents (a) and (b) may be obtained by neutralizing some of the acidgroups on the random copolymer with metal ions. Here, specific examplesof metal ions for neutralizing the acid groups include Na⁺, K⁺, Li⁺,Zn⁺⁺, Cu⁺⁺, Mg⁺⁺, Ca⁺⁺, Co⁺⁺, Ni⁺⁺ and Pb⁺⁺. Of these, preferred use canbe made of, for example, Na⁺, L⁺, Zn⁺⁺ and Mg⁺⁺. Moreover, from thestandpoint of improving resilience, the use of Na⁺ is recommended. Thedegree of neutralization of the random copolymer by these metal ions isnot subject to any particular limitation. Such neutralization productsmay be obtained by a known method. For example, use may be made of amethod in which neutralization is carried out with a compound such as aformate, acetate, nitrate, carbonate, bicarbonate, oxide, hydroxide oralkoxide of the above-mentioned metal ions.

Sodium ion-neutralized ionomer resins may be suitably used as the abovemetal ion neutralization products of the random copolymers to increasethe melt flow rate (MFR) of the material. In this way, adjustment of thematerial to the subsequently described optimal melt flow rate is easy,enabling the moldability to be improved.

Commercially available products may be used as above components (a) and(b). Illustrative examples of the random copolymer in component (a)include Nucrel N1560, Nucrel N1214, Nucrel N1035 and Nucrel AN4221C (allproducts of DuPont-Mitsui Polychemicals Co., Ltd.), and Escor 5200,Escor 5100 and Escor 5000 (all products of ExxonMobil Chemical).Illustrative examples of the random copolymer in component (b) includeNucrel AN4311, Nucrel AN4318 and Nucrel AN4319 (all products ofDuPont-Mitsui Polychemicals Co., Ltd.), and Escor ATX325, Escor ATX320and Escor ATX310 (all products of ExxonMobil Chemical).

Illustrative examples of the metal ion neutralization product of therandom copolymer in component (a) include Himilan 1554, Himilan 1557,Himilan 1601, Himilan 1605, Himilan 1706 and Himilan AM7311 (allproducts of DuPont-Mitsui Polychemicals Co., Ltd.), Surlyn 7930 (E.I.DuPont de Nemours & Co.), and Iotek 3110 and Iotek 4200 (both productsof ExxonMobil Chemical). Illustrative examples of the metal ionneutralization product of the random copolymer in component (b) includeHimilan 1855, Himilan 1856 and Himilan AM7316 (all products ofDuPont-Mitsui Polychemicals Co., Ltd.), Surlyn 6320, Surlyn 8320, Surlyn9320 and Surlyn 8120 (all products of E.I. DuPont de Nemours & Co.), andIotek 7510 and Iotek 7520 (both products of ExxonMobil Chemical).Sodium-neutralized ionomer resins that are suitable as the metal ionneutralization product of the random copolymer include Himilan 1605,Himilan 1601 and Himilan 1555.

When preparing the above-described base resin, component (a) andcomponent (b) are admixed in a weight ratio of generally between 100:0and 0:100, preferably between 100:0 and 25:75, more preferably between100:0 and 50:50, even more preferably between 100:0 and 75:25, and mostpreferably 100:0. If too little component (a) is included, the moldedmaterial obtained therefrom may have a decreased resilience.

The processability of the base resin can be further improved by, inaddition to adjusting the above mixing ratio, also adjusting the mixingratio between the random copolymers and the metal ion neutralizationproducts of the random copolymers. In this case, it is recommended thatthe weight ratio of the random copolymers to the metal ionneutralization products of the random copolymers be set to generallybetween 0:100 and 60:40, preferably between 0:100 and 40:60, morepreferably between 0:100 and 20:80, and even more preferably 0:100. Theaddition of too much random copolymer may lower the uniformity of thepellet composition.

A non-ionomeric thermoplastic elastomer (e) may be included in the baseresin so as to enhance even further both the feel of the ball at impactand the rebound. Examples of this component (e) include olefinelastomers, styrene elastomers, polyester elastomers, urethaneelastomers and polyamide elastomers. In this invention, to furtherincrease the rebound, it is preferable to use a polyester elastomer oran olefin elastomer. The use of an olefin elastomer composed of athermoplastic block copolymer which includes crystalline polyethyleneblocks as the hard segments is especially preferred.

A commercially available product may be used as component (e).Illustrative examples include Dynaron (JSR Corporation) and thepolyester elastomer Hytrel (DuPont-Toray Co., Ltd.).

Component (e) may be included in an amount of more than 0. The upperlimit in the amount included per 100 parts by weight of the base resin,although not subject to any particular limitation, is preferably notmore than 100 parts by weight, more preferably not more than 60 parts byweight, even more preferably not more than 50 parts by weight, and mostpreferably not more than 40 parts by weight. Too much component (e) maylower the compatibility of the mixture, possibly resulting in asubstantial decline in the durability of the golf ball.

Next, a fatty acid or fatty acid derivative having a molecular weight ofat least 228 but not more than 1500 may be added as component (c) to thebase resin. Compared with the base resin, this component (c) has a verylow molecular weight and, by suitably adjusting the melt viscosity ofthe mixture, helps in particular to improve the flow properties.Moreover, component (c) includes a relatively high content of acidgroups (or derivatives thereof), and is capable of suppressing anexcessive loss of resilience.

The molecular weight of the fatty acid or fatty acid derivative ofcomponent (c) may be set to at least 228, preferably at least 256, morepreferably at least 280, and even more preferably at least 300. Theupper limit may be set to not more than 1500, preferably not more than1000, more preferably not more than 600, and even more preferably notmore than 500. If the molecular weight is too low, the heat resistancecannot be improved. On the other hand, if the molecular weight is toohigh, the flow properties cannot be improved.

Preferred use as the fatty acid or fatty acid derivative of component(c) may likewise be made of, for example, an unsaturated fatty acid (orderivative thereof) containing a double bond or triple bond on the alkylmoiety, or a saturated fatty acid (or derivative thereof) in which thebonds on the alkyl moiety are all single bonds. In either case, it isrecommended that the number of carbons on the molecule be preferably atleast 18, more preferably at least 20, even more preferably at least 22,and most preferably at least 24. It is recommended that the upper limitbe preferably not more than 80, more preferably not more than 60, evenmore preferably not more than 40, and most preferably not more than 30.Too few carbons may make it impossible to improve the heat resistanceand may also make the acid group content so high as to diminish theflow-improving effect on account of interactions with acid groupspresent in the base resin. On the other hand, too many carbons increasesthe molecular weight, which may keep a distinct flow-improving effectfrom appearing.

Specific examples of the fatty acid of component (c) include myristicacid, palmitic acid, stearic acid, 12-hydroxystearic acid, behenic acid,oleic acid, linoleic acid, linolenic acid, arachidic acid and lignocericacid. Preferred use can be made of stearic acid, arachidic acid, behenicacid and lignoceric acid in particular.

The fatty acid derivative of component (c) is exemplified by metallicsoaps in which the proton on the acid group of the fatty acid has beenreplaced with a metal ion. Examples of the metal ion include Na⁺, Li⁺,Ca⁺⁺, Mg⁺⁺, Zn⁺⁺, Mn⁺⁺, Al⁺⁺⁺, Ni⁺⁺, Fe⁺⁺, Fe⁺⁺⁺, Cu⁺⁺, Sn⁺⁺, Pb⁺⁺ andCo⁺⁺. Of these, Ca⁺⁺, Mg⁺⁺ and Zn⁺⁺ are especially preferred.

Specific examples of fatty acid derivatives that may be used ascomponent (c) include magnesium stearate, calcium stearate, zincstearate, magnesium 12-hydroxystearate, calcium 12-hydroxystearate, zinc12-hydroxystearate, magnesium arachidate, calcium arachidate, zincarachidate, magnesium behenate, calcium behenate, zinc behenate,magnesium lignocerate, calcium lignocerate and zinc lignocerate. Ofthese, magnesium stearate, calcium stearate, zinc stearate, magnesiumarachidate, calcium arachidate, zinc arachidate, magnesium behenate,calcium behenate, zinc behenate, magnesium lignocerate, calciumlignocerate and zinc lignocerate are preferred.

Use may also be made of known metallic soap-modified ionomers (see, forexample, U.S. Pat. No. 5,312,857, U.S. Pat. No. 5,306,760 andInternational Disclosure WO 98/46671) when using above-describedcomponents (a) and/or (b), and component (c).

The amount of component (c) included per 100 parts by weight of theresin components when above components (a), (b) and (e) have beensuitably blended may be set to at least 5 parts by weight, preferably atleast 10 parts by weight, more preferably at least 20 parts by weight,and even more preferably at least 30 parts by weight. The upper limit inthe amount included may be set to not more than 120 parts by weight,preferably not more than 115 parts by weight, more preferably not morethan 110 parts by weight, and even more preferably not more than 100parts by weight. If the amount of component (c) included is too small,the melt viscosity may decrease, lowering the processability. On theother hand, if the amount included is too large, the durability maydecrease.

A basic inorganic metal compound capable of neutralizing acid groups inthe base resin and in component (c) may be added as component (d). Incases where this component (d) is not included and a metal soap-modifiedionomer resin (e.g., any of the metal soap-modified ionomer resins citedin the above-mentioned patent publications) is used alone, the metallicsoap and un-neutralized acid groups present on the ionomer resin undergoexchange reactions during mixture under heating, generating a largeamount of fatty acid. Because the fatty acid has a low thermal stabilityand readily vaporizes during molding, it may cause molding defects.Moreover, if the fatty acid deposits on the surface of the moldedmaterial, it may substantially lower paint film adhesion or have otherundesirable effects such as lowering the resilience of the resultingmolded material.

To solve this problem, a basic inorganic metal compound whichneutralizes the acid groups present in the base resin and component (c)is included as component (d). By including component (d), the acidgroups in the base resin and component (c) are neutralized. Moreover,synergistic effects from the blending of these respective componentsconfer the resin composition with a number of excellent properties;namely, the resin composition has a higher thermal stability and at thesame time is imparted with a good moldability, and the resilience as agolf ball-forming material is enhanced.

Illustrative examples of the metal ions used in the basic inorganicmetal compound include Li⁺, Na⁺, K⁺, Ca⁺⁺, Mg⁺⁺, Zn⁺⁺, Al⁺⁺⁺, Ni⁺⁺,Fe⁺⁺, Fe⁺⁺⁺, Cu⁺⁺, Mn⁺⁺, Sn⁺⁺, Pb⁺⁺ and Co⁺⁺. Known basic inorganicfillers containing these metal ions may be used as the basic inorganicmetal compound. Specific examples include magnesium oxide, magnesiumhydroxide, magnesium carbonate, zinc oxide, sodium hydroxide, sodiumcarbonate, calcium oxide, calcium hydroxide, lithium hydroxide andlithium carbonate. In particular, a hydroxide or a monoxide isrecommended. Calcium hydroxide and magnesium oxide, which have a highreactivity with the base resin, are more preferred. Calcium hydroxide isespecially preferred.

The amount of component (d) included per 100 parts by weight of theresin component may be set to at least 0.1 part by weight, preferably atleast 0.5 part by weight, more preferably at least 1 part by weight, andeven more preferably at least 1.2 parts by weight. The upper limit inthe amount included may be set to not more than 17 parts by weight,preferably not more than 15 parts by weight, more preferably not morethan 10 parts by weight, and even more preferably not more than 5 partsby weight. Too little component (d) fails to improve thermal stabilityand resilience, whereas too much instead lowers the heat resistance ofthe golf ball-forming material due to the presence of excess basicinorganic metal compound.

By blending specific respective amounts of components (c) and (d) withthe resin component, i.e., the base resin containing specific respectiveamounts of components (a) and (b) in admixture with optional component(e), a material having excellent thermal stability, flow properties andmoldability can be obtained, in addition to which the resilience ofmoldings obtained therefrom can be markedly improved.

It is recommended that the material formulated from specific amounts ofthe above-described resin component and components (c) and (d) have ahigh degree of neutralization (i.e., that the material be highlyneutralized). Specifically, it is recommended that at least 50 mol %,preferably at least 60 mol %, more preferably at least 70 mol %, andeven more preferably at least 80 mol %, of the acid groups in thematerial be neutralized. Highly neutralizing the acid groups in thematerial makes it possible to more reliably suppress the exchangereactions that cause trouble when only a base resin and a fatty acid orfatty acid derivative are used as in the above-cited prior art, thuspreventing the generation of fatty acid. As a result, the thermalstability is substantially improved and the processability is good,making it possible to obtain molded products of much better resiliencethan prior-art ionomer resins.

“Degree of neutralization,” as used herein, refers to the degree ofneutralization of acid groups present within the mixture of the baseresin and the fatty acid or fatty acid derivative serving as component(c), and differs from the degree of neutralization of the ionomer resinitself when an ionomer resin is used as the metal ion neutralizationproduct of a random copolymer in the base resin. When a mixture of theinvention having a certain degree of neutralization is compared with anionomer resin alone having the same degree of neutralization, becausethe material of the invention contains a very large number of metal ionsowing to the inclusion of component (d), the density of ionic crosslinkswhich contribute to improved resilience is increased, making it possibleto confer the molded product with an excellent resilience.

The resin material should preferably have a melt flow rate (MFR)adjusted within a specific range in order to ensure flow properties thatare particularly suitable for injection molding, and thus improvemoldability. In this case, it is recommended that the melt flow rate, asmeasured in general accordance with JIS-K7210 at a temperature of 190°C. and under a load of 21.18 N (2.16 kgf), be adjusted to preferably atleast 0.6 g/10 min, more preferably at least 0.7 g/10 min, even morepreferably at least 0.8 g/10 min, and most preferably at least 2 g/10min. It is recommended that the upper limit be adjusted to preferablynot more than 20 g/10 min, more preferably not more than 10 g/10 min,even more preferably not more than 5 g/10 min, and most preferably notmore than 3 g/10 min. Too high or low a melt flow rate may result in asubstantial decline in processability.

Commercial products may be used as the envelope layer-forming materials.Specific examples include those having the trade names HPF 1000, HPF2000, HPF AD1027, HPF AD1035 and HPF AD1040, as well as the experimentalmaterial HPF SEP1264-3, all produced by E.I. DuPont de Nemours & Co.

Next, the intermediate layer is described.

The intermediate layer has a material hardness, expressed as the Shore Dhardness (measured value obtained with a type D durometer in accordancewith ASTM D2240), which, while not subject to any particular limitation,is preferably at least 55, more preferably at least 60, and even morepreferably at least 63. There is no particular upper limit, although thematerial hardness of the intermediate layer may be set to preferably notmore than 75, more preferably not more than 70, and even more preferablynot more than 68. If the intermediate layer material is softer than theabove range, the ball rebound on full shots may be too low, as a resultof which an increased distance may not be achieved. On the other hand,if this material is harder than the above range, the durability of theball to cracking on repeated impact may worsen or the ball may have toohard a feel when played with a putter and on short approach shots.

The intermediate layer has a thickness which, while not subject to anyparticular limitation, is preferably at least 0.5 mm, more preferably atleast 0.9 mm, and even more preferably at least 1.0 mm. There is noparticular upper limit, although the thickness of the intermediate layermay be set to preferably not more than 2.5 mm, more preferably not morethan 1.7 mm, and even more preferably not more than 1.4 mm. If theintermediate layer thickness is too small, the durability to cracking onrepeated impact and the low-temperature durability may worsen. On theother hand, if it is too large, the feel at impact may become too hardor the envelope layers and core will be made softer to achieve ahardness balance for the ball as a whole, which may result in the ballhaving a lower initial velocity on actual shots when struck with a W#1and thus failing to achieve a sufficient distance.

Materials which may be used in the intermediate layer are not subject toany particular limitation. However, because of their high rigidity andhigh resilience, the use of ionomer resins is most preferred. Suchionomer resins are exemplified by, in particular, ionomer resins inwhich some of the carboxylic acids (i.e., acid groups) in a copolymer ofan α-olefin and an α,β-unsaturated carboxylic acid of 3 to 8 carbons areneutralized with metal ions, ionomer resins in which at least some ofthe carboxylic acids in a terpolymer of an α-olefin, an α,β-unsaturatedcarboxylic acid of 3 to 8 carbons and an α,β-unsaturated carboxylic acidester are neutralized with metal ions, and mixtures thereof.

The α-olefin in the ionomer resin is preferably ethylene or propylene.Examples of the α,β-unsaturated carboxylic acid include acrylic acid,methacrylic acid, fumaric acid, maleic acid and crotonic acid, withacrylic acid and methacrylic acid being especially preferred. Examplesof the α,β-unsaturated carboxylic acid ester include the methyl, ethyl,propyl, n-butyl and isobutyl esters of acrylic acid, methacrylic acid,fumaric acid and maleic acid. Acrylic acid esters and methacrylic acidesters are especially preferred. Examples of the metal ions whichneutralize the acid groups on the copolymer include Na⁺, K⁺, Li⁺, Zn⁺⁺,Ca⁺⁺, Mg⁺⁺, Al⁺⁺⁺ and Nd⁺⁺⁺. From the standpoint of rebound anddurability, Na⁺, Li⁺ and Zn⁺⁺ are preferred.

The content of unsaturated carboxylic acid (acid content) in the ionomerresin, although not subject to any particular limitation, may be set toa high acid content of preferably at least 16 wt %, more preferably atleast 17 wt %, and even more preferably at least 18 wt %. There is noparticular upper limit in the acid content, although the acid contentmay be set to not more than 22 wt %, preferably not more than 20 wt %,and more preferably not more than 19 wt %.

A single ionomer resin having a high acid content of at least 16 wt %may be used alone, or two or more such ionomer resins may be usedtogether, as the above-described ionomer resin of the intermediatelayer. When two or more are used together, by making joint use ofionomer resins neutralized with different metal ions, furtherimprovements in the rebound and durability to cracking under repeatedimpact can be achieved.

A commercially available product may be used as the intermediatelayer-forming material. Specific examples include AM7317, AM7318 andAM7315 (all products of DuPont-Mitsui Polychemicals Co., Ltd.), andS9150, S8150 and S8220 (all products of E.I. DuPont de Nemours & Co.).

Commonly used additives, such as pigments, fillers for adjusting thespecific gravity, dispersants, antioxidants, ultraviolet absorbers andlight stabilizers, may be suitably included in the above intermediatelayer-forming material.

In this invention, although not subject to any particular limitation,from the standpoint of keeping marks and the like that arise during useof the ball from becoming conspicuous, it is preferable to form thecover serving as the outermost layer of a resin material having a highdegree of transparency. To this end, it is preferable for theintermediate layer to include a given amount of titanium oxide in orderto block the color of the underlying material. Here, titanium oxide maybe included in just the amount needed to block the color of theunderlying material. The amount of titanium oxide included, although notsubject to any particular limitation, may be set to at least 0.5 part byweight, preferably at least 1 part by weight, and more preferably atleast 2 parts by weight, per 100 parts by weight of the resin component.There is no particular upper limit in the amount of titanium oxideincluded, although this amount may be set to not more than 10 parts byweight, preferably not more than 6 parts by weight, and more preferablynot more than 4 parts by weight. The specific gravity of the material,although not subject to any particular limitation, may be set to atleast 0.92, preferably at least 0.96, and more preferably at least 0.97.There is no particular upper limit in the specific gravity, although thespecific gravity may be set to not more than 1.15, preferably not morethan 1.05, and more preferably not more than 1.00. If the amount oftitanium oxide included is small and the specific gravity is low, it maynot be possible to block the color of the underlying material, as aresult of which the ball appearance may become darker. On the otherhand, if the amount of titanium oxide added is large and the specificgravity is too high, the rebound may decrease and a sufficient distancemay not be achieved.

To increase adhesion between the intermediate layer formed of theabove-described material and the polyurethane used in the subsequentlydescribed cover, it is desirable to abrade the surface of theintermediate layer prior to forming the cover. In addition, the adhesioncan be further enhanced by applying a primer (adhesive) to the surfaceof the intermediate layer following such abrasion treatment or by addingan adhesion reinforcing agent to the intermediate layer-formingmaterial. Examples of adhesion reinforcing agents that may beincorporated in the material include organic compounds such as1,3-butanediol and trimethylolpropane, and oligomers such aspolyethylene glycol and polyhydroxy polyolefin oligomers. The use oftrimethylolpropane or a polyhydroxy polyolefin oligomer is especiallypreferred. Illustrative examples of commercially available productsinclude trimethylolpropane produced by Mitsubishi Gas Chemical Co., Ltd.and polyhydroxy polyolefin oligomers produced by Mitsubishi ChemicalCorporation (under the trade name “Polytail H”; number of main-chaincarbons, 150 to 200; with hydroxyl groups at the ends).

Next, the cover is described. As used herein, the term “cover” denotesthe outermost layer of the ball construction, and excludes what arereferred to herein as the intermediate layer and the envelope layers.

The cover has a material hardness, expressed as the Shore D hardness,which, while not subject to any particular limitation, may be set topreferably at least 30, more preferably at least 33, and even morepreferably at least 36. There is no particular upper limit, although thematerial hardness of the cover may be set to preferably not more than50, more preferably not more than 47, and even more preferably not morethan 44. At a cover material hardness lower than this range, the spinrate on shots with a driver (W#1) may become too high or the rebound maybecome low, as a result of which an increased distance may not beachieved. On the other hand, at a cover material hardness higher thanthis range, on approach shots, the ball may lack spin receptivity andthus may have an inadequate controllability even when played by aprofessional or other skilled golfer, or the cover may have a poordurability (poor scuff resistance when hit with a wedge). As notedabove, it is critical for the material hardness of the cover to be equalto or lower than the average hardness of the core.

The thickness of the cover, while not subject to any particularlimitation, may be set to preferably at least 0.2 mm, more preferably atleast 0.3 mm, and even more preferably at least 0.4 mm. There is noparticular upper limit in the cover thickness, although the thicknessmay be set to preferably not more than 1.0 mm, more preferably not morethan 0.9 mm, and even more preferably not more than 0.8 mm. If the coveris thicker than the above range, the ball may have too low a rebound onshots with a driver (W#1), as a result of which an increased distancemay not be achieved. On the other hand, if the cover is thinner than theabove range, the ball may have a poor scuff resistance or may have aninadequate controllability in the short game, even when played by aprofessional or other skilled golfer.

The cover material, as with the above-described envelope layers andintermediate layer, is formed primarily of any of various types of resinmaterials. Although not subject to any particular limitation, from thestandpoint of controllability and scuff resistance, use may be made of amaterial selected from among thermoplastic polyurethanes, thermosetpolyurethanes and polyureas. Of these, from the standpoint of massproductivity, preferred use may be made of a thermoplastic polyurethane.

In the present invention, it is especially preferable to use a specificthermoplastic polyurethane composition composed primarily of (A) athermoplastic polyurethane and (B) a polyisocyanate compound. This resinblend is described below.

This resin composition is composed primarily of (A) a thermoplasticpolyurethane and (B) a polyisocyanate compound. Specifically, it isrecommended that the total weight of components (A) and (B) combined bepreferably at least 600, and more preferably at least 700, of theoverall weight of the cover layer.

First, the thermoplastic polyurethane (A) is described. Thisthermoplastic polyurethane includes in the structure thereof softsegments made of a polymeric polyol that is a long-chain polyol(polymeric glycol), and hard segments made of a chain extender and apolyisocyanate compound. Here, the long-chain polyol used as a startingmaterial is not subject to any particular limitation, and may be anythat is used in the prior art relating to thermoplastic polyurethanes.Exemplary long-chain polyols include polyester polyols, polyetherpolyols, polycarbonate polyols, polyester polycarbonate polyols,polyolefin polyols, conjugated diene polymer-based polyols, castoroil-based polyols, silicone-based polyols and vinyl polymer-basedpolyols. These long-chain polyols may be used singly or as combinationsof two or more thereof. Of the long-chain polyols mentioned here,polyether polyols are preferred because they enable the synthesis ofthermoplastic polyurethanes having a high rebound resilience andexcellent low-temperature properties.

Illustrative examples of the above polyether polyol includepoly(ethylene glycol), poly(propylene glycol), poly(tetramethyleneglycol) and poly(methyltetramethylene glycol) obtained by thering-opening polymerization of cyclic ethers. The polyether polyol maybe used singly or as a combination of two or more thereof. Of the above,poly(tetramethylene glycol) and/or poly(methyltetramethylene glycol) arepreferred.

It is preferable for these long-chain polyols to have a number-averagemolecular weight in the range of 1,500 to 5,000. By using a long-chainpolyol having a number-average molecular weight within this range, golfballs which are made of a thermoplastic polyurethane composition andhave excellent properties such as rebound and manufacturability can bereliably obtained. The number-average molecular weight of the long-chainpolyol is more preferably in the range of 1,700 to 4,000, and even morepreferably in the range of 1,900 to 3,000.

As used herein, “number-average molecular weight of the long-chainpolyol” refers to the number-average molecular weight computed based onthe hydroxyl number measured in accordance with JIS-K1557.

Chain extenders that may be suitably used include those employed in theprior art relating to thermoplastic polyurethanes. For example,low-molecular-weight compounds which have a molecular weight of 400 orless and bear on the molecule two or more active hydrogen atoms capableof reacting with isocyanate groups are preferred. Illustrative,non-limiting, examples of the chain extender include 1,4-butyleneglycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol and2,2-dimethyl-1,3-propanediol. Of these chain extenders, aliphatic diolshaving 2 to 12 carbons are preferred, and 1,4-butylene glycol is morepreferred.

The polyisocyanate compound is not subject to any particular limitation;preferred use may be made of one that is used in the prior art relatingto thermoplastic polyurethanes. Specific examples include one or moreselected from the group consisting of 4,4′-diphenylmethane diisocyanate,2,4-toluene diisocyanate, 2,6-toluene diisocyanate, p-phenylenediisocyanate, xylylene diisocyanate, naphthylene-1,5-diisocyanate,tetramethylxylene diisocyanate, hydrogenated xylylene diisocyanate,dicyclohexylmethane diisocyanate, tetramethylene diisocyanate,hexamethylene diisocyanate, isophorone diisocyanate, norbornenediisocyanate, trimethylhexamethylene diisocyanate and dimer aciddiisocyanate. Depending on the type of isocyanate used, the crosslinkingreaction during injection molding may be difficult to control. In thepractice of the invention, to provide a balance between stability at thetime of production and the properties that are manifested, it is mostpreferable to use 4,4′-diphenylmethane diisocyanate, which is anaromatic diisocyanate.

It is most preferable for the thermoplastic polyurethane serving asabove component A to be a thermoplastic polyurethane synthesized using apolyether polyol as the long-chain polyol, using an aliphatic diol asthe chain extender, and using an aromatic diisocyanate as thepolyisocyanate compound. It is desirable, though not essential, for thepolyether polyol to be a polytetramethylene glycol having anumber-average molecular weight of at least 1,900, for the chainextender to be 1,4-butylene glycol, and for the aromatic diisocyanate tobe 4,4′-diphenylmethane diisocyanate.

The compounding ratio of active hydrogen atoms to isocyanate groups inthe above polyurethane-forming reaction can be adjusted within adesirable range so as to make it possible to obtain a golf ball which iscomposed of a thermoplastic polyurethane composition and has variousimproved properties, such as rebound, spin performance, scuff resistanceand manufacturability. Specifically, in preparing a thermoplasticpolyurethane by reacting the above long-chain polyol, polyisocyanatecompound and chain extender, it is desirable to use the respectivecomponents in proportions such that the amount of isocyanate groups onthe polyisocyanate compound per mole of active hydrogen atoms on thelong-chain polyol and the chain extender is from 0.95 to 1.05 moles.

No particular limitation is imposed on the method of preparing thethermoplastic polyurethane used as component A. Production may becarried out by either a prepolymer process or a one-shot process inwhich the long-chain polyol, chain extender and polyisocyanate compoundare used and a known urethane-forming reaction is effected. Of these, aprocess in which melt polymerization is carried out in a substantiallysolvent-free state is preferred. Production by continuous meltpolymerization using a multiple screw extruder is especially preferred.

Commercially available products may be used as above component A.Illustrative examples include Pandex T8295, Pandex T8290, Pandex T8260and Pandex T8283 (all available from DIC Bayer Polymer, Ltd.).

Next, concerning the polyisocyanate compound used as component B, it iscritical that, in at least some portion thereof, all the isocyanategroups on the molecule remain in an unreacted state prior to injectionmolding. That is, polyisocyanate compound in which all the isocyanategroups on the molecule remain in a completely free state must be presentin the resin blend prior to injection molding. Such a polyisocyanatecompound may be present together with polyisocyanate compound in whichonly one end of the molecule is in a free state.

Various types of isocyanates may be employed without particularlimitation as the polyisocyanate compound. Illustrative examples includeone or more selected from the group consisting of 4,4′-diphenylmethanediisocyanate, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate,p-phenylene diisocyanate, xylylene diisocyanate,naphthylene-1,5-diisocyanate, tetramethylxylene diisocyanate,hydrogenated xylylene diisocyanate, dicyclohexylmethane diisocyanate,tetramethylene diisocyanate, hexamethylene diisocyanate, isophoronediisocyanate, norbornene diisocyanate, trimethylhexamethylenediisocyanate and dimer acid diisocyanate. Of the above group ofisocyanates, the use of 4,4′-diphenylmethane diisocyanate,dicyclohexylmethane diisocyanate and isophorone diisocyanate ispreferable in terms of the balance between the influence onprocessability of, for example, the rise in viscosity accompanying thereaction with the thermoplastic polyurethane serving as component A andthe physical properties of the resulting golf ball cover material.

In the cover of the inventive golf ball, although not an essentialconstituent, a thermoplastic elastomer other than the above-describedthermoplastic polyurethane may be included as component C together withcomponents A and B. Including this component C in the above resin blendenables the flow properties of the resin blend to be further improvedand enables improvements to be made in various properties required ofgolf ball cover materials, such as resilience and scuff resistance.

Illustrative examples of thermoplastic elastomers which may be used ascomponent C include Hytrel 3046, Hytrel 4047, Hytrel 4767, Hytrel 5557and Hytrel 4001 (all products of DuPont-Toray Co., Ltd.), and Dynaron6100P, Dynaron 6200P and Dynaron 4600P (all products of JSRCorporation).

Various additives may be optionally included in the above-describedcover material. Exemplary additives include pigments, dispersants,antioxidants, ultraviolet absorbers, ultraviolet stabilizers, partingagents, plasticizers and inorganic fillers (e.g., zinc oxide, bariumsulfate, titanium dioxide).

In the present invention, to render marks which arise when the ball ishit with an iron or a wedge less conspicuous, although not subject toany particular limitation, it is preferable for the cover material to begiven a high degree of transparency. Also, titanium oxide may beincluded in the cover material so as to adjust the specific gravity. Itis recommended that the amount of titanium oxide included be set to theminimum required from the standpoint of achieving a balance between thespecific gravity and the transparency. Specifically, it is recommendedthat the amount of titanium oxide included per 100 parts by weight ofthe resin component be set to preferably not more than 4.0 parts byweight, more preferably not more than 1.0 part by weight, and even morepreferably 0 part by weight (no addition). The specific gravity of thematerial, although not subject to any particular limitation, may be setto at least 0.95, preferably at least 1.00, and more preferably at least1.10. There is no particular upper limit in the specific gravity,although the specific gravity may be set to not more than 1.20,preferably not more than 1.15, and more preferably not more than 1.13.Setting the specific gravity lower than the above range makes itnecessary to mix in an ionomer resin or the like having a low specificgravity, which may worsen the scuff resistance. On the other hand,setting the specific gravity higher than the above range requires theaddition of a large amount of titanium oxide, which may render moreconspicuous any marks that form when the ball is struck with an iron ora wedge.

The color of the above-described cover material, although not subject toany particular limitation, may be varied according to user preferencesand the like. For example, a fluorescent pigment or fluorescent dye thatis yellow, orange, red, blue, pink or green may be suitably added.

Thickness Relationships among Inner Envelope Layer, IntermediateEnvelope Layer, Outer Envelope Layer, Intermediate Layer and Cover

In the present invention, although the thicknesses of the three envelopelayers are not subject to any particular limitations, in general it ispreferable that they satisfy the condition

inner envelope layer thickness≦intermediate envelope layer thickness≦

outer envelope layer thickness,

and more preferable that they satisfy the condition

inner envelope layer thickness<intermediate envelope layer thickness<

outer envelope layer thickness.

Moreover, the ratios

(inner envelope layer thickness)/(intermediate envelope layer thickness)

and

(intermediate envelope layer thickness)/(outer envelope layer thickness)

are each preferably at least 1.0, more preferably at least 1.1, and evenmore preferably at least 1.2. The upper limit may be set to preferablynot more than 1.5, more preferably not more than 1.4, and even morepreferably not more than 1.3. When the thicknesses of the respectivelayers do not satisfy the above conditions, the rebound on shots with adriver (W#1) may be inadequate, which may make it impossible to achievea good distance.

Also, although not subject to any particular limitation, it ispreferable to form the intermediate layer so as to have a largerthickness than the cover. In this case, the (intermediate layerthickness)/(cover thickness) value may be set to preferably at least1.3, more preferably at least 1.5, and even more preferably at least1.7. There is no particular upper limit in this value, although it maybe set to preferably not more than 4.0, more preferably not more than3.0, and even more preferably not more than 2.5. When the relationshipbetween the intermediate layer thickness and the cover thickness fallsoutside of the above range, the ball may take on more spin thannecessary on shots with a driver (W#1) or the initial velocity maydecrease, as a result of which an increased distance may not beachieved.

Moreover, in the overall ball which includes the cover and the core,from the standpoint of the distance achieved on shots with a driver(W#1), it is most preferable for the following relationship to besatisfied:

cover thickness<intermediate layer thickness<(outer envelope layerthickness+intermediate envelope layer thickness+inner envelope layerthickness (total envelope layer thickness))<core diameter,

and also for the following relationship to be satisfied:

cover thickness<intermediate layer thickness<inner envelope layerthickness<intermediate envelope layer thickness<outer envelope layerthickness<core diameter.

Moreover, it is recommended that the following relationship besatisfied:

(cover thickness+intermediate layer thickness)<(outer envelope layerthickness+intermediate envelope layer thickness+inner envelope layerthickness (total envelope layer thickness)).

If the cover is thicker than the intermediate layer, the rebound of theball may decrease, as a result of which an increased distance may not beachieved. If the envelope layer is thinner than the intermediate layer,a suitable spin rate may not be obtained on shots with a driver (W#1),as a result of which the desired distance may not be achieved. Moreover,it is preferable for the total envelope layer thickness to be greaterthan (cover thickness+intermediate layer thickness). When this is notthe case, a suitable spin rate may not be obtained on shots with adriver (W#1), as a result of which the desired distance may not beachieved.Hardness Relationships among Core Surface, Envelope Layers, IntermediateLayer and Cover

In this invention, it is critical for the material hardness (Shore D) ofthe cover and the average core hardness (Shore D) to satisfy therelationship

cover material hardness≦average core hardness, and for one of the innerlayers (i.e., the envelope layers and the intermediate layer) to beformed so as to be harder than the cover material hardness and/or theaverage core hardness. In this case, the difference between the materialhardness of the cover and the average core hardness (cover materialhardness−average core hardness), although not subject to any particularlimitation, may be set to preferably −1 or below, and more preferably −5or below. There is no particular lower limit, although the differencemay be set to preferably at least −15, and more preferably at least −10.If this relationship is not satisfied, designing the ball so that asuitable spin arises on full shots with a driver becomes difficult, inaddition to which the rebound may be too low, as a result of which asufficient distance may not be achieved.

Moreover, although not subject to any particular limitation, it isrecommended that the ball as a whole preferably satisfy the relationship

cover material hardness<intermediate layer material hardness>outerenvelope layer material hardness>core center hardness,

and more preferably satisfy the relationship

cover material hardness<intermediate layer material hardness>outerenvelope layer material hardness>intermediate envelope layer materialhardness>inner envelope layer material hardness>core center hardness.

In cases where the above relationships are not satisfied, energy loss bythe ball on full shots with a driver may be too large, lowering theinitial velocity; the spin rate may be too large, as a result of which agood distance may not be achieved; the ball may not be receptive to spinon approach shots, resulting in a poor controllability; and the covermay have a poor durability.

Multi-piece solid golf balls having the above-described core, envelopelayers, intermediate layer and cover can be manufactured by a knownprocess such as injection molding. More specifically, a multi-piecesolid golf ball having a six-layer construction can be obtained by usingpress molding or injection molding to fabricate a core composedprimarily of a rubber material, using specific injection-molding moldsto successively form envelope layers and an intermediate layer aroundthe core, then injection-molding a cover material over the resultingintermediate layer-encased sphere. Alternatively, another method offorming the cover may be used in which a pair of half-cups are moldedbeforehand using the above-described cover material, the intermediatelayer-encased sphere is enclosed in these half-cups, and molding underapplied pressure is carried out at from 120 to 170° C. for 1 to 5minutes.

In the golf ball of the invention, as in conventional golf balls, inorder to further improve the aerodynamic properties and thereby increasethe distance traveled by the ball, it is desirable to form a pluralityof dimples on the surface of the cover. By optimizing dimple parameters,such as the types and total number of dimples, owing to synergisticeffects with the above-described ball construction, the trajectory ismore stable, making it possible to obtain a golf ball having anexcellent distance performance. Moreover, the cover may be subjected tovarious types of treatment, such as surface preparation, stamping andpainting in order to enhance the design and durability of the golf ball.

First, the total number of dimples, although not subject to anyparticular limitation, may be set to preferably at least 280, morepreferably at least 300, and even more preferably at least 320. Theupper limit may be set to preferably not more than 360, more preferablynot more than 350, and even more preferably not more than 340. If thenumber of dimples is higher than the above range, the ball trajectorymay become lower, possibly decreasing the distance traveled by the ball.On the other hand, if the number of dimples is lower than the aboverange, the ball trajectory may become higher, as a result of which anincreased distance may not be achieved.

The shapes of the dimples are not limited to circular shapes; one ormore type from among, for example, various polygonal shapes, dewdropshapes and oval shapes may be suitably selected. In cases where, forexample, circular dimples are used, the diameter of the dimples may beset to at least about 2.5 mm but not more than about 6.5 mm, and thedepth may be set to at least 0.08 mm but not more than 0.30 mm.

To fully exploit the aerodynamic characteristics of the dimples, thedimple coverage on the spherical surface of the golf ball, which is thesum of the individual dimple surface areas, each defined by the borderof the flat plane circumscribed by the edge of that dimple, expressed asa ratio (SR) with respect to the spherical surface area of the ball wereit to be free of dimples, is preferably at least 60% but not more than900. Also, to optimize the trajectory of the ball, the value V₀ obtainedby dividing the spatial volume of each dimple below the flat planecircumscribed by the edge of that dimple by the volume of a cylinderwhose base is the flat plane and whose height is the maximum depth ofthe dimple from the base is preferably at least 0.35 but not more than0.80. In addition, the VR value, which is the sum of the volumes of eachindividual dimple formed below the flat plane circumscribed by the edgeof that dimple, as a percentage of the volume of the ball sphere were itto have no dimples thereon, is preferably at least 0.6% but not morethan 1.0%. Outside the above ranges for these values, the ball mayassume a trajectory that is not conducive to achieving a good distance,as a result of which the ball may fail to travel a sufficient distancewhen played.

The golf ball of the invention, which can be manufactured so as toconform with the Rules of Golf for competitive play, may be produced toa ball diameter which is of a size such that the ball does not passthrough a ring having an inside diameter of 42.672 mm, but is not morethan 42.80 mm, and to a weight of generally from 45.0 to 45.93 g.

As explained above, by having the envelope layer composed of threelayers—an inner envelope layer, an intermediate envelope layer and anouter envelope layer, and by optimizing the respective thicknesses andhardnesses of the envelope layers, intermediate layer and cover in themanner described above, the golf ball of the invention is highlybeneficial for professionals and other skilled golfers, both because ithas an excellent flight performance and also excellent controllabilityin the short game that are acceptable to such golfers, and also becauseit has a good feel at impact and an excellent scuff resistance.

EXAMPLES

Examples of the invention and Comparative Examples are given below byway of illustration, and not by way of limitation.

Examples 1 and 2, Comparative Examples 1 to 6 Formation of Core

Rubber compositions were formulated as shown in Table 1, then molded andvulcanized at 155° C. for 21 minutes to form cores.

TABLE 1 Example 1 Example 2 Formulation Polybutadiene A 80 80 (pbw)Polybutadiene B 20 20 Zinc acrylate 43.3 53.7 Organic peroxide 1.05 1.05Antioxidant 0.2 0.2 Zinc oxide 84.4 82.6 Sulfur 0.085 0.085 Zinc salt ofpentachlorothiophenol 0.4 0.4 Zinc stearate 5 5 Sulfur/Organic peroxide(weight ratio) 0.08 0.08

The chief materials in Table 1 are described below.

-   Polybutadiene A: Available under the trade name “BR730” from JSR    Corporation.-   Polybutadiene B: Available under the trade name “BR51” from JSR    Corporation.-   Organic peroxide: Dicumyl peroxide, available under the trade name    “Percumyl D” from NOF Corporation.-   Antioxidant: 2,2′-Methylenebis(4-methyl-6-t-butylphenol), available    under the trade name “Nocrac NS-6” from Ouchi Shinko Chemical    Industry Co., Ltd.-   Zinc stearate: Available under the trade name “Zinc Stearate G” from    NOF Corporation.

Formation of Envelope Layers, Intermediate Layer and Cover

Next, an inner envelope layer, an intermediate envelope layer, an outerenvelope layer, an intermediate layer and a cover, each formulated asshown in Table 2, were successively injection-molded over the coreobtained above, thereby producing a multi-piece solid golf ball having asix-layer construction in which three envelope layers, an intermediatelayer and a cover are formed over the core. The dimples shown in FIG. 2were formed at this time on the cover surface. Details on the dimplesare given in Table 3.

TABLE 2 No. No. No. 1 No. 3 No. 5 No. 6 10 11 Formulation HPF 1000 100(pbw) AM7317 50 AM7318 50 AN4319 100 20 AN4221C 80 Magnesium 100 60stearate Magnesium 2.8 1.7 oxide Trimethyl- 1.1 olpropane T-8283 100 65T-8290 35 Hytrel 4001 15 15 Titanium oxide 3 Polyethylene 1.5 1.5 waxIsocyanate 9 9 compound Yellow 1.5 1.5 fluorescent pigment

The chief materials in Table 2 are described below.

-   HPF 1000: An HPF resin available from E.I. DuPont de Nemours & Co.-   AM7317, AM7318: High-stiffness ionomers available from DuPont-Mitsui    Polychemicals Co., Ltd.-   AN4319, AN4221C: Available under the trade name “Nucrel” from    DuPont-Mitsui Polychemicals Co., Ltd.-   Magnesium oxide: Available under the trade name “Kyowamag MF150”    from Kyowa Chemical Industry Co., Ltd.-   T-8283, T-8290: MDI-PTMG type thermoplastic polyurethanes available    under the trade name “Pandex” from DIC Bayer Polymer.-   Hytrel 4001: A polyester elastomer available from DuPont-Toray Co.,    Ltd.-   Polyethylene wax: Available under the trade name “Sanwax 161P” from    Sanyo Chemical Industries, Ltd.-   Isocyanate compound: 4,4′-Diphenylmethane diisocyanate.-   Yellow fluorescent pigment:    -   Available under the trade name “FZ-2815 Yellow” from Sinloihi        Co., Ltd.

TABLE 3 Number of Diameter Depth No. dimples (mm) (mm) V_(o) SR VR 1 184.6 0.13 0.53 81.6 0.819 2 234 4.5 0.14 0.53 3 42 3.7 0.14 0.53 4 12 3.30.13 0.53 5 6 3.0 0.16 0.53 6 14 3.5 0.14 0.53 Total 326

Dimple Definitions

-   Diameter: Diameter of flat plane circumscribed by edge of dimple.-   Depth: Maximum depth of dimple from flat plane circumscribed by edge    of dimple.-   V₀: Spatial volume of dimple below flat plane circumscribed by    dimple edge, divided by volume of cylinder whose base is the flat    plane and whose height is the maximum depth of dimple from the base.-   SR: Sum of individual dimple surface areas, each defined by the flat    plane circumscribed by the edge of the dimple, as a percentage of    surface area of ball sphere were it to have no dimples thereon.    (units: %)-   VR: Sum of volumes of individual dimples formed below flat plane    circumscribed by the edge of the dimple, as a percentage of volume    of ball sphere were it to have no dimples thereon. (units: %)

The golf balls obtained were each tested and evaluated by the methodsdescribed below with regard to properties of the various layers, such asthickness, hardness and deflection, and also flight performance andscuff resistance. The results are shown in Tables 4 and 5. Allmeasurements were carried out in a 23° C. atmosphere.

(1) Core Deflection (mm)

The core was placed on a hard plate, and the amount of deformation bythe core when compressed under a final load of 1,275 N (130 kgf) from aninitial load state of 98 N (10 kgf) was measured.

(2) Core Surface Hardness

The durometer indenter was set substantially perpendicular to thespherical surface of the core, and JIS-C hardness measurements (inaccordance with JIS-K6301) were taken at two randomly selected points onthe core surface. The average of the two measurements was used as thecore surface hardness. This hardness was designated as the core surfacehardness H.

In addition, the Shore D hardness of the core surface was measured bythe same method as just described, but using a type D durometer inaccordance with ASTM-2240.

(3) Core Cross-Sectional Hardness

The core was cut in half, creating a flat plane. The durometer indenterwas set substantially perpendicular at the center thereof, and the JIS-Chardness was measured (in accordance with JIS-K6301).

The cross-sectional hardness was measured at the following places.

-   -   C0: center of core    -   C2: A position 2 mm from center of core    -   C4: A position 4 mm from center of core    -   C6: A position 6 mm from center of core    -   C8: A position 8 mm from center of core    -   C10: A position 10 mm from center of core    -   C12: A position 12 mm from center of core

In addition, the Shore D hardness at the center of the core wasmeasured. The Shore D hardness was measured by the same method asdescribed above, but using a type D durometer in accordance withASTM-2240.

(4) Material Hardnesses of Envelope Layers, Intermediate Layer and Cover

The respective layer-forming materials were molded into sheets having athickness of about 2 mm and held for two weeks at 23° C., followingwhich the sheets were stacked to a thickness of at least 6 mm and thehardnesses were measured with a type D durometer in accordance with ASTMD-2240.

(5) Flight Performance on Shots with a Driver

The distance traveled by the ball when hit at a head speed (HS) of 50m/s with a driver (abbreviated below as “W#1”; TourStage X-Drive 460,manufactured by Bridgestone Sports Co., Ltd.; loft angle, 8.5°) mountedon a golf swing robot was measured. The results were rated according tothe criteria shown below. The spin rate was the value measured for theball, using an apparatus for measuring initial conditions, immediatelyafter the ball was hit in the same way as described above.

Good: Carry was 225 m or more

NG: Carry was less than 225 m

(6) Spin Rate on Approach Shots

The spin rate of a ball hit at a head speed (HS) of 20 m/s with a sandwedge (abbreviated below as “SW”; TourStage TW-01, manufactured byBridgestone Sports Co., Ltd.) mounted on a golf swing robot wasmeasured. The results were rated according to the criteria shown below.The spin rate was measured in the same way as described above; that is,immediately after impact, and using an apparatus for measuring initialconditions.

Good: Spin rate of 6,000 rpm or more

NG: Spin rate of less than 6,000 rpm

(7) Scuff Resistance

A non-plated pitching sand wedge was set in a swing robot and the ballwas hit once at a head speed of 40 m/s, following which the surfacestate of the ball was visually examined and rated as follows.

Exc: No significant damage

Good: Still usable

NG: No longer usable

(8) Feel on Shots with a Driver and on Approach Shots

Sensory tests were carried out by ten golfers having a head speed of 48m/s or more who place much importance on the distance traveled by theball on shots with a driver (W#1). The feel of the ball at impact wasrated according to the following criteria.

Shots with a Driver (W#1)

-   -   Good: Seven or more of the ten golfers thought the ball had a        good solid feel indicative of take-off    -   NG: Too soft (three or fewer of the ten golfers thought the ball        had a good feel)

Approach Shots

-   -   Good: Seven or more of the ten golfers thought the ball had a        soft feel indicative of good controllability    -   NG: Feel at impact was hard and unpleasant (three or fewer of        the ten golfers thought the ball had a good feel)

TABLE 4 Example 1 2 Core Diameter (mm) 27.4 27.4 Weight (g) 16.5 16.5Deflection (mm) 3.2 2.5 Shore D Center hardness (Shore D) 38 38 hardnessSurface hardness (Shore D) 56 62 Average hardness (Shore D) 47 50Surface hardness − Center hardness (Shore D) 17 24 JIS-C Center hardnessC0 (JIS-C) 61 61 hardness Cross-sectional hardness C2 (JIS-C) 61 61Cross-sectional hardness C4 (JIS-C) 61 61 Cross-sectional hardness C6(JIS-C) 62 62 Cross-sectional hardness C8 (JIS-C) 64 64 Cross-sectionalhardness C10 (JIS-C) 66 69 Cross-sectional hardness C12 (JIS-C) 70 72Surface hardness H (JIS-C) 84 92 Surface hardness H − Center hardness C0(JIS-C) 23 31 Cross-sectional hardness C4 − Center hardness C0 (JIS-C) 00 Cross-sectional hardness C10 − Cross-sectional hardness C4 (JIS-C) 5 8Surface hardness H − Cross-sectional hardness C10 (JIS-C) 18 23 InnerMaterial No. 1 No. 1 envelope Thickness (mm) 1.55 1.55 layer Specificgravity 0.95 0.95 Material hardness (Shore D) 48 48 Inner envelopeDiameter (mm) 30.5 30.5 layer-encased sphere Weight (g) 20.4 20.4Intermediate Material No. 3 No. 3 envelope Thickness (mm) 2.0 2.0 layerSpecific gravity 0.96 0.96 Material hardness (Shore D) 51 51Intermediate envelope Diameter (mm) 34.5 34.5 layer-encased sphereWeight (g) 26.8 26.8 Outer Material No. 5 No. 5 envelope Thickness (mm)2.3 2.3 layer Specific gravity 0.95 0.95 Material hardness (Shore D) 5555 Outer envelope Diameter (mm) 39.1 39.1 layer-encased sphere Weight(g) 36.1 36.1 Intermediate Material No. 6 No. 6 layer Thickness (mm) 1.21.2 Specific gravity 0.97 0.97 Material hardness (Shore D) 65 65Intermediate Diameter (mm) 41.5 41.5 layer-encased sphere Weight (g)42.0 42.0 Cover Material No. 10 No. 11 Thickness (mm) 0.6 0.6 Specificgravity 1.12 1.12 Material hardness (Shore D) 40 43 Ball Diameter (mm)42.7 42.7 Weight (g) 45.5 45.5 Cover material hardness − Average corehardness (Shore D) −7 −7

TABLE 5 Example 1 Example 2 Flight W#1 Spin rate (rpm) 1957 2033performance Carry (m) 227.1 228.3 Rating Good Good SW Spin rate (rpm)6452 6382 Rating Good Good Feel W#1 Good Good Approach shots Good GoodScuff resistance Exc Exc

1. A multi-piece solid golf ball comprising a core, an envelope layerencasing the core, an intermediate layer encasing the envelope layer,and a cover which encases the intermediate layer and has formed on asurface thereof a plurality of dimples, wherein the envelope layer iscomprised of an inner envelope layer, an intermediate envelope layer andan outer envelope layer; the inner, intermediate and outer envelopelayers, the intermediate layer and the cover are each formed primarilyof a resin material which may be of the same or different types; thecore is formed primarily of a rubber material; the cover has a materialhardness (Shore D) and the core has a center hardness (Shore D) and asurface hardness (Shore D) with an arithmetic mean thereof (average corehardness), which cover material hardness and average core hardnesssatisfy the following condition:cover material hardness≦average core hardness; one layer from among theenvelope layers and the intermediate layer has a material hardness(Shore D) which is higher than either or both of the cover materialhardness (Shore D) and the average core hardness (Shore D); and, lettingC10 represent a JIS-C cross-sectional hardness at a position 10 mm froma center of the core on a cross-section obtained by cutting the core inhalf and H represent a JIS-C surface hardness of the core, thehardnesses C10 and H satisfy the following condition:10≦(H−C10)≦30.
 2. The multi-piece solid golf ball of claim 1, whereinthe intermediate envelope layer is formed so as to be harder than theinner envelope layer and to have a material hardness difference (ShoreD) with the inner envelope layer of from 1 to 10, and so as to be softerthan the outer envelope layer and to have a material hardness difference(Shore D) with the outer envelope layer of from 1 to
 10. 3. Themulti-piece solid golf ball of claim 1, wherein the intermediate layerand the cover have thicknesses which satisfy the following condition:1.3≦intermediate layer thickness/cover thickness≦4.0.
 4. The multi-piecesolid golf ball of claim 1, wherein the inner envelope layer,intermediate envelope layer and outer envelope layer have thicknesseswhich satisfy the following condition:inner envelope layer thickness≦intermediate envelope layerthickness≦outer envelope layer thickness.
 5. The multi-piece solid golfball of claim 1, wherein the intermediate layer is formed of a materialwhich includes an ionomer resin having an acid content of at least 16 wt%.
 6. The multi-piece solid golf ball of claim 1, wherein the corecenter, outer envelope layer, intermediate layer and cover havehardnesses (Shore D) which satisfy the following condition:cover material hardness<intermediate layer material hardness>outerenvelope layer material hardness>core center hardness.
 7. Themulti-piece solid golf ball of claim 1, wherein the core center, innerenvelope layer, intermediate envelope layer, outer envelope layer,intermediate layer and cover have hardnesses (Shore D) which satisfy thefollowing condition:cover material hardness<intermediate layer material hardness>outerenvelope layer material hardness>intermediate envelope layer materialhardness>inner envelope layer material hardness>core center hardness. 8.The multi-piece solid golf ball of claim 1, wherein the core, innerenvelope layer, intermediate envelope layer, outer envelope layer,intermediate layer and cover have thicknesses which satisfy thefollowing condition:cover thickness<intermediate layer thickness<(outer envelope layerthickness+intermediate envelope layer thickness+inner envelope layerthickness)<core diameter.
 9. The multi-piece solid golf ball of claim 1,wherein the inner envelope layer, intermediate envelope layer, outerenvelope layer, intermediate layer and cover have thicknesses whichsatisfy the following condition:(cover thickness+intermediate layer thickness)<(outer envelope layerthickness+intermediate envelope layer thickness+inner envelope layerthickness).
 10. The multi-piece solid golf ball of claim 1, wherein atleast one layer from among the inner envelope layer, intermediateenvelope layer and outer envelope layer is formed of a material obtainedby blending: an ionomer resin component of (a) an olefin-unsaturatedcarboxylic acid random copolymer and/or a metal ion neutralizationproduct of an olefin-unsaturated carboxylic acid random copolymer mixedwith (b) an olefin-unsaturated carboxylic acid-unsaturated carboxylicacid ester random terpolymer and/or a metal ion neutralization productof an olefin-unsaturated carboxylic acid-unsaturated carboxylic acidester random terpolymer in a weight ratio between 100:0 and 0:100, and(e) a non-ionomeric thermoplastic elastomer in a weight ratio between100:0 and 50:50.
 11. The multi-piece solid golf ball of claim 1, whereinat least one layer from among the inner envelope layer, intermediateenvelope layer and outer envelope layer is formed of a material obtainedby blending as essential components: 100 parts by weight of a resincomponent composed of, in admixture, a base resin of (a) anolefin-unsaturated carboxylic acid random copolymer and/or a metal ionneutralization product of an olefin-unsaturated carboxylic acid randomcopolymer mixed with (b) an olefin-unsaturated carboxylicacid-unsaturated carboxylic acid ester random terpolymer and/or a metalion neutralization product of an olefin-unsaturated carboxylicacid-unsaturated carboxylic acid ester random terpolymer in a weightratio between 100:0 and 0:100, and (e) a non-ionomeric thermoplasticelastomer in a weight ratio between 100:0 and 50:50; (c) from 5 to 120parts by weight of a fatty acid and/or fatty acid derivative having amolecular weight of from 228 to 1500; and (d) from 0.1 to 17 parts byweight of a basic inorganic metal compound capable of neutralizingun-neutralized acid groups in the base resin and component (c).
 12. Themulti-piece solid golf ball of claim 11 wherein at least two layers fromamong the inner envelope layer, intermediate envelope layer and outerenvelope layer are formed of the material of claim
 11. 13. Themulti-piece solid golf ball of claim 11 wherein the inner envelopelayer, intermediate envelope layer and outer envelope layer are allformed of the material of claim
 11. 14. The multi-piece solid golf ballof claim 1, wherein the core has a deflection when compressed under afinal load of 1,275 N (130 kgf) from an initial load state of 98 N (10kgf) of at least 1.8 mm but not more than 6.0 mm.
 15. The multi-piecesolid golf ball of claim 1, wherein the cover is formed by injectionmolding a resin blend composed primarily of (A) a thermoplasticpolyurethane and (B) a polyisocyanate compound, which resin blendcontains a polyisocyanate compound in at least some portion of which allthe isocyanate groups remain in an unreacted state.
 16. The multi-piecesolid golf ball of claim 1, wherein, letting C0 represent a JIS-Ccross-sectional hardness at the center of the core on a cross-sectionobtained by cutting the core in half, C4 represent a JIS-Ccross-sectional hardness at a position 4 mm from the center of the core,C10 represent a JIS-C cross-sectional hardness at a position 10 mm fromthe center of the core, and H represent a JIS-C surface hardness of thecore, the hardnesses C0, C4, C10 and H satisfy the conditions:C4−C0≦5,3≦C10−C4≦12, andC4−C0<C10−C4<H−C10.
 17. The multi-piece solid golf ball of claim 1,wherein the core has a single-layer structure.